On Sunday 04 August 2002 14:23, Chris
Forbes-Ewan wrote:
> A couple of months ago I made the innocent (I thought) statement that Alpha Centauri is a triple-star system, >with Proxima Centauri being the closest of the three stars to our solar system. An astronomy graduate told >me that this is incorrect--rather, Proxima Centauri is not part of the system, but is an entirely separate star, so >Alpha Centauri is actually a double-star system.
> Does anyone (e.g. Ian Musgrave) know the current position on this?
Zero Sum replied
All I can say is what I was taught in childhood and have never heard contradicted.Ian Musgrave replied 4/8/02Alpha and Beta are a clear binary system but Proxima (which is currently closest to us and has been so for a long time and will be so for a long time yet) rotates (I have avoided the word orbit to avoid being divisive) around their (common) centre of gravity at such a huge distance as to render the point moot.
Calling Alpha/Beta a binary is not incorrect. Calling Alpha/Beta/Proxima a trinary system is also not incorrect.
It is a bit like the 'argument' about Pluto being a planet or a Kuiper belt object. There are a few rabid people about it being one or the other. Most people realise the definition is arbitrary.
If you argue that Alpha/Beta/Proxima are (and can only be described as) a trinary system, then there are not too many stars that are not part of binary systems. The galaxy is one hell of a melee...
There is no standard such as "if two stars rotate around each other more than once per galactic revolution they are a binary system" (that I know of). Perhaps there needs to be.
>Ian, is there any rule that determines the limits of what is considered gravitationally bound?
They have to orbit a common centre of
mass. Alpha centauri A and B orbit a common centre of mass with a
period of 80 years, proxima and the Alpha centuri AB system orbit a
common centre of mass with a period of around 500,000 years. Obviously
there are going to be ambiguous cases, where it is difficult to
determine if two stars have a long orbital period, or are just passing
by on trajectories that look superficially orbit like. All we can do is
measure their positions for longer, until we can determine a good orbit
or not.
Binary/trinary etc star systems can be
embedded in higher order systems (eg in globular and open clusters) so
determining what is/is not part of the system may be problematic.
>If you had two suns in orbit around the glactic centre, you could place them in orbits such that they circled each other each galactic revolution without consideration of their gravitaional attraction to each other. Would this be an appropriate measure of binding? That they circle each other more than once in a galactic rotation?
No. In the opposite ends of the galaxy
case it is clear that the gravitational attraction of each star for each
other is negligible, and that they are in fact orbiting the galactic
centre of mass.
At 07:59 8/08/02 +1000, Chris
Forbes-Ewan wrote:
[snip] >"So alpha centauri is not an observational triple star. But it is certainly a gravitational triple star with proxima orbiting the close binary pair of alpha1 and alpha2. My aim was to dispel people's expectation of seeing three stars when they turn their scopes onto alpha centauri. They will only see two." I think that clears the matter up.Yes, it certainly does. Also, proxima centauri is MUCH MUCH fainter than Alpha 1 and 2, so unless you know what you are looking for, you can't tell it from any other faint star in the telescopic field.
Cheers! Ian
